1. Field of the Invention
The present invention relates to an electrically conductive polymer material which contains a polyaniline or derivative thereof doped with a protonic acid, and a method for its production. By the method according to the invention, a conductive polymer material is obtained having substantially reduced acidity, sufficiently high conductivity, and which is suitable, for example, for melt-processing in conventional plastics processing apparatus.
2. Description of the Related Art
Electrically conductive polymers are at present subject to great interest in different parts of the world. These polymers can be used for replacing metal conductors and semi-conductors in a number of applications, such as batteries, sensors, switches, light cells, circuit boards, heating elements, electrostatic discharge elimination (ESD), and electromagnetic interference shielding (EMI). The advantages of conductive polymers over metals include their light weight, mechanical properties, corrosion resistance, and less expensive synthesis and processing methods.
Electrically conductive plastics can be divided roughly into two categories: filled conductive plastics, in which a conductive filler, such as carbon black or soot, carbon fiber, metal powder, etc., is added to a thermosetting or thermoplastic resin, and intrinsically conductive plastics, which are based on polymers which have been rendered electrically conductive by oxidation, reduction or protonation (doping).
The electrical conductivity of filled conductive polymers is dependent on mutual contacts between the conductive filler particles. Usually a well dispersed filler is needed in amounts of approximately 10-50 wt. % to produce composites having a good conductance. However, such conductive composites involve problems: their mechanical and certain of their chemical properties are crucially impaired as the filler content increases and the polymer content decreases; their conductivity is difficult to control, especially within the semiconductor range; and stable and homogenous dispersing of the filler into the matrix plastic is difficult.
Intrinsically conductive plastics can be prepared from organic polymers containing long conjugated chains formed by double bonds and heteroatoms. The polymers can be rendered conductive by modifying the .pi.- and .pi.-p-electron systems in their double bonds and heteroatoms by adding to the polymer certain blending or doping agents which will serve as electron receptors or electron donors in the polymer. Thereby electron holes or extra electrons are formed in the polymer chain, enabling electric current to travel along the conjugated chain.
An advantage of the intrinsically conductive plastics is the ease of varying their conductivity as a function of the amount of the doping agent, i.e. the degree of doping, especially within low conductivity ranges. On the other hand, achieving low conductivities with filled conductive plastics is difficult. Examples of currently known intrinsically conductive polymers include polyacetylene, poly-p-phenylene, polypyrrole, polythiophene and its derivatives, and polyaniline and its derivatives.
There are two principal methods for processing polymers into the desired pieces, fibers, films, etc., i.e. melt processing and solution processing. Melt processing is a versatile processing method, whereas solution processing is suitable mainly for the preparation of fibers and films but not profiled pieces. However, the processing and doping of most intrinsically conductive polymers involve problems with respect to the handling, stability, homogeneity, etc., of the materials.
Polyaniline, with its derivatives, is in particular a technically and commercially promising intrinsically conductive polymer. An aniline polymer or a derivative thereof is made up of aniline monomers or derivatives thereof, the nitrogen atom of which is bonded to the para-carbon of the benzene ring of the subsequent unit. Unsubstituted polyaniline may appear in a number of forms, including leucoemeraldine, protoemeraldine, emeraldine, nigraline, and toluprotoemeraldine forms. For conductive polymer applications, the emeraldine form is generally used, having the formula ##STR1## wherein X is approximately 0.5.
According to state-of-the-art technology, the doping of polyaniline is usually carried out by using protonic acids, which include HCl, H.sub.2 SO.sub.4, HNO.sub.3, HClO.sub.4, HBF.sub.4, HPF.sub.6, HF, phosphoric acids, sulfonic acids, picrinic acid, n-nitrobenzoic acid, dichloroacetic acid, and polymer acids. Preferably the doping is carried out using sulfonic acid or its derivatives, such as dodecylbenzenesulfonic acid (DBSA). The protonization is focused on the iminic nitrogen atoms in the aniline units according to the formula presented above, which comprise approximately 50% of the N atoms of polyaniline. Examples of publications in the field include U.S. Pat. Nos. 3,963,498, 4,025,463, and 4,983,322, which are hereby incorporated by reference. The doping of polyaniline with protonic acids is also widely discussed in the literature in the field. U.S. Pat. No. 5,171,478, which is hereby incorporated by reference, discloses a method for increasing the molar mass of polyaniline by heating the polyaniline until its viscosity has increased.
U.S. Pat. No. 5,232,631, which is hereby incorporated by reference, discloses processible polyaniline compositions and blends that exhibit much lower percolation thresholds, sometimes even below 1% w/w, of conductive polyaniline. The patent relates to conductive polymers and particularly to the use of functionalized protonic acids to induce processibility of electrically conductive polyanilines, and to induce solubility of electrically conductive polyanilines in organic liquids or fluid (melt) phases of solid polymers.
However, the processing a doped polyaniline having good conductivity properties by versatile melt-processing methods has involved problems, to which solutions have been sought. The mixture of polyaniline or a derivative thereof and a doping protonic acid is an indefinite, staining, strongly corrosive, viscous, fluid or paste-like, dispersion-type material which is difficult to handle. A significant improvement to the preparation of a polyaniline-based, melt-processable conductive polymer having good conductivity and other properties is disclosed in EP Patent Application 545729, which is hereby incorporated by reference. According to the invention disclosed in the said Application, a solid, electrically conductive plastics material with good properties is obtained by first contacting the polyaniline or a derivative thereof with a doping protonic acid and by thereafter heat-treating the reaction product or mixture at a temperature of approximately +40.degree. to +250.degree. C. The heat treatment results in a change of the above-described preprotonized mixture of polyaniline or a derivative thereof and a doping protonic acid, from a mixture that is difficult to handle into a homogenous, solid, relatively inert powder or granular material that is easy to handle and is suitable for use in, for example, various melt processing methods.
Polyaniline doped with a functionalized protonic acid has proved to be especially usable when it contains an excess of the protonic acid, such as the above-mentioned sulfonic acid or a derivative thereof, i.e. the mixture contains an amount of acid not only sufficient for the doping, but also for the plasticization of the mixture. When protonic acid is used in this manner in excess, effective doping of polyaniline is obtained and, additionally, the doped polyaniline becomes a material suitable for melt processing, since the protonic acid plays the above-mentioned two roles in the mixture. When protonic acid is used in this manner in excess, the result is a doped polyaniline which has an acidic pH value. However, acidity encumbers the use of the conductive polymer in most applications. In addition to acidic products corroding and contaminating other composite plastics components and the environment, acidic material to be processed damages the equipment, such as melt-processing apparatus, by corroding it. These disadvantages result in few practical applications for an otherwise good conductive material.
State-of-the-art publications have almost completely overlooked the problem of acidity. On the other hand, it is evident in the state of the art that mere mixing together of a polyaniline of the emeraldine base form and a doping agent such as dodecylbenzenesulfonic acid will not suffice to produce a homogenous mixture; the result is the above-mentioned mixture or dispersion which is difficult to handle. In state-of-the-art publications, sufficient mixing and doping has in general been effected by dissolving the parts of the mixture in the same solvent. An example of the state-of-the-art publications concerning this is U.S. Pat. No. 5,006,278, which is hereby incorporated by reference. In addition, the methods disclosed in the state-of-the-art publications in general use solution processing, cf. WO Patents 8901694 and 9013601, in which case the acidity problem will not cause as much harm as in melt processing methods for which the polyaniline-based conductive material according to the present invention is especially suitable. In addition, it can be stated that acidity is not especially problematic in all applications, such as accumulators and batteries.
One way of improving the processibility and of reducing the acidity of a conductive polymer material which contains polyaniline doped with a protonic acid, preferably sulfonic acid and most preferably dodecylbenzenesulfonic acid, is disclosed in EP Patent Application 582919, which is hereby incorporated by reference. According to the method disclosed in this publication, a certain additive is used for the neutralization of polyaniline or its derivative doped with a protonic acid, and also for its plasticization and/or stabilization. According to the method of this publication, a mixture containing polyaniline doped with a protonic acid can be rendered more plastic, more stable and more neutral by adding to the mixture a metal compound or by treating the doped polyaniline with a metal compound. The metal compound can be selected from among numerous alternatives, according to the desired property to be emphasized. The compounds may be oxides, hydroxides, halides, or equivalent.
In the invention disclosed, it is noted that the most advantageous are the compounds of zinc, of which zinc oxide (ZnO) has been found to be the most suitable. According to a preferred embodiment, the metal compound is allowed to react first with any acid which, together with the metal compound, forms a compound which substantially stabilizes and plasticizes doped polyaniline. The acid reacting with the metal compound need not be the same as the protonic acid to be used for the doping of polyaniline. However, using as the acid reacting with the metal compound the same protonic acid as is used for the doping of the polyaniline has proved to be a preferred embodiment. The mixing of the ingredients is carried out preferably in a melt mixing apparatus, for example by means of a kneader, a compounder, or a screw mixer, at a temperature of approximately 50.degree.-200.degree. C. However, this method has the disadvantage that a metal compound of the type disclosed therein will strongly decrease the conductivity of polyaniline at higher concentrations of the metal compound.